As the search for, and production of, mixtures of hydrocarbons and water moves into greater depths, the practical limit has been reached for drilling and production platforms anchored to the sea bottom. In deeper water, we will have to accept platforms which are mounted on flotation structures subjected to all six marine motions.
Separation of the well stream components must be carried out in vessels mounted on the flotation structures and, therefore, are subjected to the disruptive force of marine motions. All the conventional vessels utilized in separating these well stream fluids are being studied for modification in view of this marine environment.
Although the present invention will be reduced to practice in connection with oil and gas wells, the broader consideration of other mixutres of fluids of different densities are contemplated. Keep in mind that the hydrocarbon fluids are representative of all mixtures of fluids of different densities in this art.
Platform and ship-mounted process equipment must function properly under various amounts of marine movement. The worst part of a storm at sea may only last for 30 minutes, but if it causes significant process upset (such as carryover of oil out the gas line), then the entire facility may have to be shut down. Restarting a complex facility can require eight hours, which is a significant loss of production and revenue. Process vessels must be small due to offshore weight and space considerations, yet they must be able to function during the frequent storms which occur at sea. The answer to this difficult set of constraints is well designed vessel internals.
U.S. Pat. No. 4,073,734, Neely E. Lowrie, issued Feb. 14, 1978, has a disclosure including efficiently separating oil, water and gas from a subsea well. Although the 1978 structure was efficient for two-phase separation, additional structure is needed for three-phase separation subjected to marine motions. To understand the differences, the teachings of the 1978 patent are incorporated by reference.
Essentially, the 1978 patent provided a horizontal table or baffle extending substantially the length of the horizontally extended vessel. The height of the horizontal table or baffle was placed approximately at the expected interface between the oil and water strata within the separator. The water, with some oil, entered two compartments below the table, one in each half of the separator, through a centrally located well. The water entered the two compartments through apertures in the well wall leading into each compartment. The oil that entered the two compartments returned to the oil body above the horizontal baffle through vertical pipes penetrating the baffle. The water left the vessel through two outlets located near each end of the separator. The oil was removed from the vessel after it flowed over a vertical weir located at the far end of the separator from the inlet and above the horizontal baffle.
Further study of the 1978 disclosure and accumulation of additional facts and experience since the issue of that patent have indicated that significant improvements can be made. The sloshing of the liquids in and out of the central well in the horizontal table disturbed the gravity-separating process of the oil and water. Additional structure could be utilized to maintain better isolation of the fluid-packed compartment beneath the table. The present disclosure utilizes an alternate arrangement of structure within the separator which will eliminate the problems or limitations of the 1978 system. A significant factor that was not adequately considered in the 1978 patent is the major problem of foam reduction or decay.